Rivnuts from Homestead, PA on 8/12/2023 9:56:22 AM:
Great question. As you probably have found, there are on-line stores that offer the traditional or similar GAP and C&O Trail jerseys. Those are at:
Of course these are not brink and mortar retail stores along the GAP/C&O Trail.
By "retailers" I presume you mean walk-in stores. That said, there are many candidates none of which I've gone into specifically looking for those jerseys and thus don't have any confirmed locations. Candidates would include sport stores, bike shops and visitor centers along the way. I can think of the following in that sense and their locations are as follows:
REI - Pittsburgh's South Side
Dick's Sporting Goods - Homestead, PA
Visitor Center - West Newton, PA
West Newton Bicycle Shop - West Newton, PA
Bikes Unlimited - Connellsville, PA
Visitor Center, Meyersdale, PA
Wheelzup at Canal Place - Cumberland, MD
C&O Bicycle - Hancock MD
I'll check out the ones between Pgh and West Newton over the next several days and let you know if I find them. I can check out the others when I pass them when I ride all the way to DC in September.